The fleet experiences an average of one 'critical' intervention every 200 miles, each time avoiding hitting a person or causing significant property damage.

Uber's fleet of self-driving prototype vehicles is reportedly making slow progress toward achieving true autonomous operation without requiring frequent human intervention.

Internal documents obtained and analyzed by Recode provide a glimpse of Uber's project statistics for autonomous pilot projects in Phoenix, Pittsburgh and San Francisco. The numbers appear to focus on the past few months, revealing assessment metrics as Uber's fleet has collectively grown from 5,000 to 18,000 miles averaged per week.

Uber tracks different types of engagements to track progress, starting with a simple count of 'disengagement' incidents when a human driver takes control for any reason. Another metric focuses on 'critical' incidents, when human intervention was required to avoid hitting pedestrians or causing $5,000 or more in property damage. Importantly, Uber is also tracking 'bad experiences' when the autonomous systems cause potentially nauseating motions or hard braking.

The report suggests humans are intervening more than once per mile driven, showing no improvement over the course of the month. It is unclear, however, if the fleet was attempting more challenging test routes during the same period.

Critical interventions have varied wildly, from as low as 40 miles per incident in early February to as high as 200 miles per incident in March, though the numbers seem to be improving over time.

Unfortunately for passengers, the 'bad experience' data shows the opposite trend. A peak of 4.5 miles per incident dates back to mid-January. The number of incidents later climbed to a steady rate of around one recurrence every two miles in early March.

The numbers show that Uber's autonomous platform still has a long way to go before customers can be picked up by a drone taxi with no human chauffeur behind the wheel. The project's fate has also been put into question after the company was sued by Waymo, an Alphabet subsidiary spun off from Google's self-driving car project.